1. Field of the Invention
The present invention relates to a cam shaft driving structure of an SOHC (Single Overhead Cam) V-type engine and, more particularly to a cam shaft driving structure for driving a cam shaft by a crank shaft via an intermediate shaft.
2. Description of the Related Art
A cam shaft driving structure of an SOHC V-type engine is generally structured such that the cam shaft provided in a cylinder head of each of cylinders inclined forward and rearward is provided with a sprocket and a crank shaft is provided with a sprocket for the cam shaft of each of the cylinders, and the sprocket of the cam shaft is directly driven by the sprocket of the crank shaft via a chain. However, in this configuration, since it is necessary to provide two sprockets on the crank shaft to drive the cam shafts, the crank shaft becomes long. Consequently, a width of the engine is increased.
As an example of a cam shaft driving structure in which a width of an overhead cam shaft V-type engine is reduced, a double overhead cam V-type engine is known, in which cam shafts of forward and rearward cylinders are driven by a crank shaft via an intermediate shaft (see Publication of Examined Patent Application No. Hei.4-75364).
In this engine, the intermediate shaft is provided with two sprockets for respectively driving the cam shaft of the forward cylinder and the cam shaft of the rearward cylinder and the crank shaft is provided with one sprocket for driving the intermediate shaft. Because one sprocket on the crank shaft can be dispensed with as compared to a case where the cam shafts of the respective cylinders are directly driven by the crank shaft, a length of the crank shaft can be reduced. Therefore, a width of the engine can be reduced.
When the SOHC V-type engine is provided with the intermediate shaft to reduce the width of the engine, the following problems arise.
1) It is difficult to optimize a direction in which a tensioner of a swingable chain guide for guiding a cam chain exerts a force with respect to a chain.
2) It is difficult to use raw members of cylinder blocks or cylinder heads to which the tensioners are to be attached as physically identical raw members, or their shapes are complex.
3) Since the type or the number of parts is increased, cost is disadvantageously high.
The present invention has been developed for obviating the above-described problem, and an object of the present invention is to provide a cam shaft driving structure of an SOHC V-type engine which can drive a cam shaft of each of cylinders by a crank shaft via an intermediate shaft to reduce a width of the engine and have interchangeable or common parts as much as possible.
To achieve the above-described problem, there is provided a cam shaft driving structure of an Single Overhead Cam V-type engine comprising: a crank shaft provided in a crank case; a first cam shaft and a second cam shaft respectively provided in a cylinder head of a first cylinder and a cylinder head of a second cylinder, the first and second cylinders being arranged in an upper portion of the crank case such that the first and second cylinders are widened in V-shape in a forward and rearward direction with the crank shaft as a center; an intermediate shaft rotatably provided in a portion of the crank case that is situated above the crank shaft and in parallel with the crank shaft; a first cam chain and a second cam chain for respectively transmitting rotation of the intermediate shaft to the first and second cam shafts; an intermediate shaft chain for transmitting rotation of the crank shaft to the intermediate shaft; a first cam chain guide and a second cam chain guide for respectively guiding a tense side of the first cam chain and a loose side of the first cam chain; and a third cam chain guide and a fourth cam chain guide for respectively guiding a tense side of the second cam chain and a loose side of the second cam chain, wherein a fulcrum of the second cam chain guide and a fulcrum of the fourth cam chain guide are provided in the vicinity of the intermediate shaft, a first tensioner for the cam chain guide associated with one of the first and second cam chains in which its loose side corresponds to its upper side is provided in one of the cylinder heads, and a second tensioner for the cam chain guide associated with one of the first and second cam chains in which its loose side corresponds to its lower side is provided in a cylinder block of the cylinder.
With this configuration, the tensioners are placed in the cylinder block and the cylinder head. Thereby, the tensioners are capable of exerting forces to the cam chains in suitable directions.
It is preferable that each of the second cam chain guide and the fourth cam chain guide is provided with two portions which respectively abut with the tensioners.
With this configuration, the cam chain guides associated with the loose sides of the cam chains are interchangeable between them.
The cam shaft driving structure of the Single Overhead Cam V-type engine may further comprise a means for positioning the first cam chain guide in a direction toward a base end thereof and a means for positioning the third cam chain guide in a direction toward a base end thereof in the crank case, wherein each of the first cam chain guide and the third cam chain guide is positioned in a direction toward a tip end thereof by a lower face of the corresponding cylinder head and is provided with two protrusions that abut with the corresponding cylinder block.
With this configuration, the chain guides associated with the tense sides of the cam chains are interchangeable between them.
Further, the cam shaft driving structure of the Single Overhead Cam V-type engine, may have the following structure: the first and second cylinders are arranged such that one of the cylinders substantially overlaps with the other cylinder except the first to fourth cam chain guides and the first and second tensioners, if the one cylinder is rotated 180 degrees about an imaginary axis orthogonal to a center axis of the crank shaft and a center axis of the intermediate shaft, each of the first and second cylinders is provided with a chain tunnel as a passage of the corresponding cam chain, the tunnel extending from a portion where a sprocket of the intermediate shaft is provided in the crank case to a portion where a sprocket of the cam shaft is provided in the corresponding cylinder head via the corresponding cylinder block, each of the cylinder head of the first cylinder and the cylinder head of the second cylinder is structured such that a boss to which the first tensioner is mounted when used in the cylinder associated with the cam chain in which its upper side corresponds to its loose side, is provided in a wall portion of the chain tunnel that is situated in the corresponding cylinder head, a first positioning portion is provided in an inner wall face of the cam chain tunnel that is situated on the same side of the wall portion where the boss is provided, for positioning one of the second and fourth cam chain guides associated with the cam chain in which its tense side corresponds to its upper side, in a direction toward the cam shaft in a longitudinal direction thereof, and a second positioning portion is provided in an inner wall face of the chain tunnel that is situated on an opposite side of the wall portion where the boss is provided, for positioning one of the second and fourth cam chain guides associated with the cam chain in which its tense side corresponds to its lower side, in a direction toward the cam shaft in a longitudinal direction thereof, and each of the cylinder block of the first cylinder and the cylinder block of the second block is structured such that a boss to which the second tensioner is mounted when used in the cylinder associated with the cam chain in which its lower side corresponds to its loose side is provided in a wall portion of the chain tunnel that is situated in a corresponding cylinder block, an inner wall face of the chain tunnel that is situated on the same side of the wall portion where the boss is provided is shaped to enable abutment with a back face of one of the second and fourth cam chain guides associated with the cam chain in which its tense side corresponds to its lower side, and an inner wall face of the cam chain tunnel that is situated on an opposite side of the wall portion where the boss is provided is shaped to enable abutment with a back face of one of the second and fourth cam chain guides associated with the cam chain in which its tense side corresponds to its upper side.
With this configuration, the raw members from which the cylinder heads are made are interchangeable between them, and the raw members from which the cylinder blocks are made are also interchangeable between them.
It is preferable that the cam shaft driving structure of the Single Overhead Cam V-type engine, further comprises: an intermediate shaft chain guide that swings about the fulcrum of one of the second and fourth cam chain guides associated with the cam chain in which its loose side corresponds to its lower side, thereby mailing contact with the loose side of the intermediate shaft chain.
With this configuration, the number of parts can be reduced.
It is preferable that a transmission ratio between rotation of the crank shaft and rotation of the intermediate shaft is substantially 1 to 1 and the sprockets of the intermediate shaft on which the first and second cam chains are installed have diameters substantially equal to or smaller than a diameter of the sprocket of the intermediate shaft on which the intermediate shaft chain is installed.
With this configuration, a wide space within an angle formed between the first and second cylinders can be utilized.
These objects as well as other objects, features and advantages of the invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.